Plastic packages have been the most widely used packages for sealing semiconductor elements.
Plastic packages are widely used for memory elements of large capacity, operational elements, and the like. Plastic packages are inexpensive but have poor heat radiating properties. Further, plastic packages do not seal completely water or moisture which may easily penetrate them.
In order to improve the hermetic sealing and heat radiating properties of packages, ceramic packages are sometimes used. A multi-layered ceramic package is formed from a laminate composed of a plurality of thin plates of alumina (Al.sub.2 O.sub.3) or mullite (Al.sub.6 Si.sub.2 O.sub.13). Metallized wiring is printed on one of the intermediate alumina plates so that the inside semiconductor chip is connected to an outside lead frame without penetrating a lead frame into the Al.sub.2 O.sub.3 or mullite plates. Such ceramic packages are very expensive because they are formed of laminations of numbers of thin ceramic plates.
In place of ceramic packages, so called "CER DIP" packages are also used. A CER DIP package constitutes a substrate of Al.sub.2 O.sub.3 or mullite, a lead frame, and a cap. The substrate, the lead frame, and the cap are sealed with PbO-B.sub.2 O.sub.3 series low melting point glass.
A package in which pins are arranged in two directions is called a DIP type package. A package in which a substrate of Al.sub.2 O.sub.3 or mullite, a lead frame, and a cap are sealed with low melting point glass is thus called a CER DIP package, and a package identical to a CER DIP except that the pins are arranged in four directions is called a "CER Quad" package. Both a CER DIP and a CER Quad are often generally called a CER DIP. The term "CER DIP package" used herein includes a CER DIP and CER Quad packages.
A CER DIP package must be distinguished from a ceramic package. Although both are made of ceramic material, a CER DIP package is sealed with glass and a ceramic package is not.
A CER DIP package has higher hermetic sealing, better heat radiating properties, and is more reliable than a plastic package. Further, a CER DIP package is inexpensive compared to a ceramic package.
Packages that have higher heat radiating properties have been developed using an Al.sub.2 O.sub.3 -lamination ceramic package in which a Cu-W cooling plate is brazed onto an element mounting portion as described in JP-A-59-56746 and JP-A-59-61948. Other packages have been developed by using a BeO substrate, because BeO has a higher heat conductivity than that of Al.sub.2 O.sub.3 as described in JP-A-50-104571. (The term "JP-A" as used herein means an unexamined published Japanese patent application.)
Packages having a Cu-W cooling plate are expensive because the cooling plate is brazed to an Al.sub.2 O.sub.3 substrate.
Packages using a BeO substrate are difficult to handle because BeO is poisonous.
AlN is known in the art as ceramics having a higher heat conductivity than Al.sub.2 O.sub.3. Packages formed using an AlN substrate would be expected to have high heat radiating properties. However, there are problems forming a low melting point glass seal on a package having an AlN substrate.
FIG. 3 shows a cross section of a conventional package having an AlN substrate sealed with low melting-point glass.
The package has an AlN substrate 1, a lead frame 4, and a cap 9. The substrate 1, the cap 9, and the lead frame 4 are sealed with low melting point glass 3. The glass 3 is PbO-B.sub.2 O.sub.3 series low melting point glass which is the same as that used in sealing the Al.sub.2 O.sub.3 package. In this package, however, a number of air bubbles 5 are formed at the boundary between the AlN substrate or the AlN cap and the low melting-point glass. Because of these air bubbles, the sealing strength of the package is lowered and the hermetic sealing is deteriorated. Water or moisture is thus able to penetrate the package and affect the semiconductor element (chip). The long term reliability of this package is conspicuously lowered.
The air bubbles 5 form because gas is generated at the boundary between the AlN substrate or the AlN cap and the low melting point glass by a chemical reaction between the liquid (molten) PbO-B.sub.2 O.sub.3 series glass and AlN. Because of the above defects, a package as shown in FIG. 3 is not actually on the market.